Heterocyclic aromaticity, concept

Heterocyclic chemistry offers a much wider prospect for exploring the possibilities and limitations of the aromaticity concept than hydrocarbon chemistry. As a result, quantitative measures for aromaticity have been devised, and improved molecular calculation methods allow for a better evaluation of the experimental data already obtained and predictions of novel aromatic heterocycles. 

This section is devoted to heterocyclic molecules isoelectronic with dications and dianions of the molecules that are central to the aromaticity concept, i.e., cyclobutadiene and benzene (Scheme 5). 1,3-Diboretene... 

Finally, the concept of aromaticity in heterocyclic chemistry is the subject of an overview by Professor V. I. Minkin and Drs. B. Ya. Simkin and M. N. Glukhovtsev of Rostov State University in Russia. This new review shows just how wide the aromaticity concept has become. The present authors have successfully extended its range to the consideration of numerous very diverse heterocyclic systems. 

In a review article (91H(32)127) the concept of heterocyclic aromaticity has been presented, summarized, and applied to discussion of tautomerism for a series of hydroxy-substituted azines. Later, the tautomerism of 2(l/7)-pyridone, 2(177)-pyrimidone, 2(177)-pyrazinone, 4-pyrimidone, and 3(277)-pyridazinone in low-temperature inert gas matrices has been investigated by means of IR spectroscopy. It was found that the relative stability of the oxo and hydroxy tautomers of these compounds depends in a systematic way on the relative position of the lactam group and the second nitrogen atom in the ring (92JPC6250). 

Subsequendy, the aromaticity concept built on carbocydes has been applied successfully to heterocyclic organic compounds, i.e., to a family of aromatic compounds which contain one or more heteroatoms as a part of their cyclic TC-electron system. However, some of the measures developed for aromatic hydrocarbons, particularly those based on geometrical parameters, have been modified to be used for heterocycles (1993JCI70, 2001T5715, 2010SY1485). 

Modern concepts have been extended to the chemistry of heterocyclic compounds more slowly than to the chemistry of aromatic and aliphatic systems, but efforts are now being made to classify and explain the properties and reactions of heterocyclic compounds in terms of these newer ideas (cf. reference 11). However, many of the most important heterocyclic compounds are potentially tautomeric, and elucidation of their tautomeric composition must precede a logical treatment of their properties. Further, many natural products such as the nucleic acids and alkaloids contain potentially tautomeric groups and information of this type is needed for a detailed explanation of th reactions which they undergo,... 

A quantitative scale of reactivity for aromatic substrates (fused, heterocyclic, and substituted rings) has been devised, based on the hard-soft concept (p. 338). From MO theory, a quantity, called activation hardness, can be calculated for each position of an aromatic ring. The smaller the activation hardness, the faster the attack at that position hence the treatment predicts the most likely orientations for incoming groups. 

Heterocyclic systems have played an important role in this historical development. In addition to pyridine and thiophene mentioned earlier, a third heterocyclic system with one heteroatom played a crucial part protonation and methylation of 4//-pyrone were found by J. N. Collie and T. Tickle in 1899 to occur at the exocyclic oxygen atom and not at the oxygen heteroatom, giving a first hint for the jr-electron sextet theory based on the these arguments.36 Therefore, F. Arndt, who proposed in 1924 a mesomeric structure for 4//-pyrone, should also be considered among the pioneers who contributed to the theory of the aromatic sextet.37 These ideas were later refined by Linus Pauling, whose valence bond theory (and the electronegativity, resonance and hybridization concepts) led to results similar to Hiickel s molecular orbital theory.38... 

Redox shuttles based on aromatic species were also tested. Halpert et al. reported the use of tetracyano-ethylene and tetramethylphenylenediamine as shuttle additives to prevent overcharge in TiS2-based lithium cells and stated that the concept of these built-in overcharge prevention mechanisms was feasible. Richardson and Ross investigated a series of substituted aromatic or heterocyclic compounds as redox shuttle additives (Table 11) for polymer electrolytes that operated on a Li2Mn40g cathode at elevated temperatures (85 The redox potentials of these... 

Aromatic compounds and their reactions are a big part of any Organic 11 course. We introduce you to the aromatic family, including the heterocyclic branch, in Chapter 6. (You may want to brush up on the concept of resonance beforehand.) Then in Chapters 7 and 8, you find out more than you ever wanted to know about aromatic substitution reactions, starring electrophiles and nucleophiles. 

Aromaticity has been long recognized as one of the most useful theoretical concepts in organic chemistry. It is essential in understanding the reactivity, structure and many physico-chemical characteristics of heterocyclic compounds. Aromaticity can be defined as a measure of the basic state of cyclic conjugated TT-electron systems, which is manifested in increased thermodynamic stability, planar geometry with non-localized cyclic bonds, and the ability to sustain an induced ring current. In contrast to aromatic compounds there exist nonaromatic and antiaromatic systems. Thus, pyrazine (69)... 

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